By way of example, with respect to such portable power working machine as chain saws, brush cutters, etc., a two-stroke gasoline engine, e.g., an approximately 25-100 cc small air-cooled internal combustion engine, is ordinarily provided within the main housing as a power source for driving the operating part, e.g., the saw chain, etc. Further, a carburetor as a fuel supplying and regulating means is provided in the intake system of such an engine, while an air cleaner for cleaning outside-air that is brought in by removing dust therefrom is disposed immediately upstream of this carburetor.
A conventional example of such an air cleaner is described below with reference to FIG. 3 where the rear portion of a chain saw is shown along with a portion of a two-stroke gasoline engine mounted thereon, the two-stroke gasoline engine being of a reverse-scavenging system having a total of four scavenging ports with two each on the left and right.
With respect to chain saw 50 shown in the diagram, a lateral L-shaped top handle 54, into which a throttle lock lever and a throttle trigger are incorporated, is so disposed as to span the upper surface part and rear part of a main housing 52. A cooling fan (not shown) driven by an engine 60 is disposed inside the main housing 52. A portion of the air brought into the main housing 52 by this cooling fan is brought into a cylinder 62 of the engine 60 via an air cleaner 10′ and carburetor 65 disposed at the rear part of the top handle 54 and main housing 52.
The engine 60 is mounted sideways within the main housing 52 with its intake port 63 on the upper side and its head part (combustion chamber) 64 facing rearward. The carburetor 65 is connected further upstream than the intake port 63 via a vibration-absorbent air pipe 66 comprising a synthetic rubber material, and the air cleaner 10′ is connected immediately upstream of the carburetor 65.
The air cleaner 10′ comprises: a synthetic resin case 11 comprising a bottom wall 15 with a substantially inverted-V-shaped cross-section [a bottom wall 15A on the side of the carburetor 65 (upper side) and a bottom wall 15B on the side of a cleaner inlet 16 (lower side)] and a perimeter side-wall 15C; a thick plate-shaped filter (filtering element) 12 that is so disposed as to cover the upper opening of the case 11; a synthetic resin cover 13 that is disposed in such a manner as to be sandwiched the filter 12 between the case 11 and the cover 13; and a screw member 18 that is screwed into an internal thread part 17 formed around a boundary between the carburetor-side bottom wall 15A of the case 11 and the cleaner inlet-side bottom wall 15B in order to attach this cover 13, wherein the cover 13 and the filter 12 may be removed by loosening the screw member 18. It is noted that the internal thread part 17 has its upper part (a boss part 17a) protrude upward from the bottom wall 15 (15A, 15B).
In addition, as can be seen in the case 11 portion of the air cleaner 10′ shown in FIG. 4, a cleaner outlet 20 is so provided in the carburetor-side bottom wall 15A of the case 11 as to be continuous with an intake port 67a of an internal passage 67 in the carburetor 65. An upper end part (a boss part 20a) of the cleaner outlet 20 protrudes slightly upward from the upper surface of the bottom wall 15A. On the bottom wall 15A is disposed a back flow guide member 19 made of a metal plate and for preventing mixed fuel (discussed later) that back flows from the carburetor 65 from traveling towards the filter 12.
As can be seen in the plan view in FIG. 5A showing an attached state and the perspective view in FIG. 5B, this guide member 19 comprises: a bent plate part 19A of an inverted lip groove shape that lies above the center of the cleaner outlet 20; and a crescent-shaped plate part 19B that extends from an upper plate part 19a of the bent plate part 19A in such a manner as to bend slightly downward towards the internal thread part 17. Screwing plate parts 19c, 19c that bend outward are respectively provided at the lower ends of both side plate parts 19b, 19b of the bent plate part 19A. Set screws 22 are screwed into the carburetor 65 via through-holes 19d respectively provided in the screwing plate parts 19c, 19c, and via insertion holes 29, 29 each with a boss part 29a that are provided in the bottom wall 15A of the case 11. The back flow guide member 19 and the case 11 are thus fastened together and affixed to the carburetor 65.
In addition, in order to define a fuel accumulation part 25 for collecting and accumulating fuel that back flows from the carburetor 65, upright plate parts 27, 28 are provided on the cleaner inlet-side bottom wall 15B of the case 11.